To be fair, the first thirty or so years of Spider-Man are solid. You'd, of course, get the occasional stupid or overly goofy storyline but nothing character ruining.
The problems came with his marriage to MJ. For some reason idiots who don't actually read the comics think that it ruins his teenage appeal, even though the last time you could reasonably call him a teenager was like 1965. They also think he needs to be some sort of free swinging bachelor that bangs every woman in his life, even though that again goes against his actual character.
To try to fix this they first did the Clone Saga. In that they tried to reveal that the Peter fans have been reading about for the last twenty years (and thus the married one) was a clone and this new, commitment-free guy (Ben Reilly) was the 'real' one. The idea there is that Peter would have lost his powers, moved away to Portland with MJ (who'd have a baby), and Ben Reilly would then become the real Spider-Man. Of course fans rejected that stupid idea, so Ben ended up being the real clone and died, Peter got his powers back, moved back to New York, and MJ eventually had a miscarriage (sorta -- originally the nurse who over saw it would've been revealed to have stole the baby and given it to Norman Osborn but the idea was scrapped, though it did lead to the alternate universe series Spider-Girl).
A couple years after the Clone Saga ended they tried it again. In what's known as the New Chapter era, the shitty writer Howard Mackie just suddenly started writing Peter and MJ as if they were teenagers again and killed off MJ in a plane crash to get Peter out of the marriage. Fans yet again rejected this (I think this was the worst selling era of all time) and everyone realized making Peter a widower made him seem even older than the marriage did. So, ultimately, it was revealed that MJ escaped the plane at the last minute and the two were eventually reunited when J. Micheal Straczynski took over Spider-Man for a while (and the first couple of years of his work were really great).
Then they tried it for a third time with One More Day. I talked about that one already in this thread. This one lasted the longest but was still largely hated. It looks like Nick Spencer, the current main Spider-Man writer, is working towards getting MJ and Peter back together.
Really, it all comes down to the people in charge going "We know better the fans and we know what they secretly, truly want, even if they won't admit it. This is for the best."